On 3/3/25 17:41, Lilith Gkini wrote:
> On Mon, Mar 03, 2025 at 12:06:58PM +0100, Vlastimil Babka wrote:
>> On 3/2/25 19:01, Lilith Persefoni Gkini wrote:
>> > If the `search` pattern is not found in the freelist then the function
>> > should return `fp == search` where fp is the last freepointer from the
>> > while loop.
>> >
>> > If the caller of the function was searching for NULL and the freelist is
>> > valid it should return True (1), otherwise False (0).
>>
>> This suggests we should change the function return value to bool :)
>>
>
> Alright, If you want to be more technical it's
> `1 (true), otherwise 0 (false).`
> Its just easier to communicate with the true or false concepts, but in C
> we usually don't use bools cause its just 1s or 0s.
Yeah, I think bools were not used initially int the kernel, but we're fine
with them now and changing a function for other reasons is a good
opportunity to modernize. There are some guidelines in
Documentation/process/coding-style.rst about this (paragraphs 16 and 17).
int is recommended if 0 means success and -EXXX for error, bool for simple
true/false which is the case here.
>> I think there's a problem that none of this will fix or even report the
>> situation properly. Even worse we'll set slab->inuse to 0 and thus pretend
>> all objects are free. This goes contrary to the other places that respond to
>> slab corruption by setting all objects to used and trying not to touch the
>> slab again at all.
>>
>> So I think after the while loop we could determine there was a cycle if (nr
>> == slab->objects && fp != NULL), right? In that case we could perform the
>> same report and fix as in the "Freepointer corrupt" case?
>
> True! We could either add an if check after the while as you said to
> replicate the "Freepointer corrupt" behavior...
> Or...
>
> I hate to say it, or we could leave the while condition with the equal
> sign intact, as it was, and change that `if` check from
> `if (!check_valid_pointer(s, slab, fp)) {`
> to
> `if (!check_valid_pointer(s, slab, fp) || nr == slab->objects) {`
You're right!
> When it reaches nr == slab->objects and we are still in the while loop
> it means that fp != NULL and therefore the freelist is corrupted (note
> that nr starts from 0).
>
> This would add fewer lines of code and there won't be any repeating
> code.
> It will enter in the "Freechain corrupt" branch and set the tail of
> the freelist to NULL, inform us of the error and it won't get a chance
> to do the nr++ part, leaving nr == slab->objects in that particular
> case, because it breaks of the loop afterwards.
>
> But it will not Null-out the freelist and set inuse to objects like you
> suggested. If that is the desired behavior instead then we could do
> something like you suggested.
We could change if (object) to if (object && nr != slab->objects) to force
it into the "Freepointer corrupt" variant which is better. But then the
message should be also adjusted depending on nr... it should really report
"Freelist cycle detected", but that's adding too many conditions just to
reuse the cleanup code so maybe it's more readable to check that outside of
the while loop after all.
